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COVID-19 antibody tests will eventually prove key to determining how and when to reopen businesses and allow mass congregations—but we’re still far from that point, says the Centers for Disease Control (CDC) in updated guidance about coronavirus antibody testing.

The agency writes that blood tests which detect COVID-19 antibodies, telltale signs that a person has contracted the disease but recovered, “should not be used to make decisions about grouping persons residing in or being admitted to congregate settings, such as schools, dormitories, or correctional facilities” and also “should not be used to make decisions about returning persons to the workplace.”

While numerous firms such as Roche, Abbott Laboratories, Quest Diagnostics, and others are rushing to make these so-called serologic tests, CDC officials note that there’s still far too much information left unknown as well as questions about the accuracy of various tests.

For instance, some COVID-19 antibody tests can provide incorrect results up to half the time through false positives or negatives, according to the CDC. And then there’s the problem of immunity. While antibodies typically indicate a defensive biological response, meaning that a recovered patient has some level of immunity to a pathogen, the novel coronavirus is so new that it’s still unclear how strong that immunity is, how long it lasts, and whether recovered patients are still at risk for reinfection.

That’s a frustrating scientific reality with which businesses and society at large will have to grapple. On Wednesday, lab testing giant Quest Diagnostics announced that it would provide coronavirus testing services to large employers, including antibody tests, to help guide safe strategies for returning employees to work.

But as the CDC guidance highlights, it may well take years—and continuous, long-term follow-up testing of the population—to truly understand which antibody tests are most accurate, and what their results even mean.